A water spiral, often seen when water drains from a sink or tub, is primarily caused by a combination of initial water movement, the shape of the container, and the effect of the Earth's rotation, known as the Coriolis force.
When water drains, it naturally converges towards the drain opening. If there is any pre-existing rotation in the water, even a very slight one from filling the basin or movement within the house, this rotation tends to increase as the water moves towards the center. This phenomenon is similar to an ice skater spinning faster as they pull their arms in.
The Role of the Coriolis Force
As the reference states, the water spiral is not imaginary but "is very real in the rotating reference frame." The Earth's rotation causes a force known as the Coriolis force. This force doesn't actually push or pull but appears to deflect moving objects when viewed from a rotating frame of reference (like the Earth's surface).
"The Coriolis force tends to make things on the surface of the object to spiral a certain direction." Since the water in your sink is on the surface of the Earth, "As the earth rotates, this motion causes everything on the surface to experience the Coriolis force, including the water in your sink."
- In the Northern Hemisphere, the Coriolis force causes fluids (like water or air) to deflect to the right.
- In the Southern Hemisphere, it causes them to deflect to the left.
This deflection contributes to the swirling motion.
Other Influencing Factors
While the Coriolis force is a fundamental contributor to large-scale spirals (like hurricanes), in small containers like sinks or tubs, other factors often have a much stronger and more immediate effect on the direction of the spiral:
- Initial Water Movement: Any residual swirl or current in the water from filling the sink or stirring can easily dominate the subtle effect of the Coriolis force.
- Basin Shape: The geometry of the sink or tub can influence how the water flows towards the drain.
- Drain Design: The shape and placement of the drain can affect the formation of the vortex.
- Temperature Gradients: Differences in water temperature can create convection currents.
These factors can easily overpower the relatively weak Coriolis effect in a small container, meaning a sink spiral doesn't always reliably swirl in the direction predicted by the hemisphere (clockwise in the North, counter-clockwise in the South). However, in carefully controlled experiments or with very large bodies of water, the Coriolis effect becomes more evident.
In essence, the water spiral is a vortex formed as water drains, influenced by its initial state, the container's properties, and the Coriolis force caused by the Earth's rotation.
